I'm reading Sipser's Introduction to the Theory of Computation (3rd edition). In chapter 0 (pg. 2), he says we don't know the answer to "what makes some problems computationally hard and others easy," however, he then states that "researchers have discovered an elegant scheme for classifying problems according to their computational difficulty. Using this scheme, we can demonstrate a method for giving evidence that certain problems are computationally hard, even if we are unable to prove that they are."
So my question is: HOW is it possible to classify problems according to their computational difficulty, if we don't even know what makes a problem computationally easy/hard in the first place?
Also, what/where is this "scheme" that does this classifying. (I did some googling and couldn't find anything)